With a view to improving service life and reliability, the brushless motor is being largely employed for various drive motors. In general, the brushless motor requires the use of a position detector for detecting the position of a movable element. However, in order to further reduce price and size, a brushless motor which does not require the use of a position detector is necessary. A conventional example of a drive device for such a brushless motor is disclosed in, for example, the Japanese Laid-open Patent Publication No. 52-80415.
Hereinafter, the above described conventional drive device for the brushless motor will be discussed with reference to the drawings.
FIG. 17 is a circuit diagram of the conventional drive circuit for the brushless motor. In FIG. 17, driving coils 1 to 3 are connected at their one end to each other. The driving coil 1 is connected at the other end to the anode of a diode 4, the cathode of a diode 5 and the respective collectors of driving transistors 10 and 13. The driving coil 2 is connected at the other end to the anode of a diode 6, the cathode of a diode 7 and the respective collectors of driving transistors 11 and 14. The driving coil 3 is connected at the other end to the anode of a diode 8, the cathode of a diode 9 and the respective collectors of driving transistors 12 and 15. The respective cathodes of the diodes 4, 6 and 8 and the respective emitters of the driving transistors 10, 11 and 13 are connected to a positive power supply line, and the respective anodes of the diodes 5, 7 and 9 and the respective emitters of the transistors 13, 14 and 15 are connected to ground. The other ends of the driving coils 1 to 3 are also connected with a filtering circuit 16 which generates an output to a power supply switching circuit 17. An output from the power supply switching circuit 17 is supplied t the respective bases of the driving transistors 10 to 15.
The drive device for the brushless motor, which is so constructed as hereinabove described, operates in the following manner.
FIG. 18 is a diagram for explaining the operation of the device shown in FIG. 17, wherein Uo, Vo and Wo represent the respective waveforms of the electric power signals supplied to the driving coils 1, 2 and 3. The power supply waveform Uo, Vo and Wo have their high harmonic components removed by the filtering circuit 16 and are respectively converted by the filtering circuit 16 into output signals F1, F2 and F3 which are delayed 90.degree. in phase. It is noted that the filtering circuit 16 is a primary filter and is constituted by, for example, a RC passive filter, a primary Miller integrator, etc., the cut-off frequency of which is set to a sufficiently low region as compared with the frequencies of the power supply waveforms across the coils. The output signals F1, F2 and F3 are inputted to the power supply switching circuit 17. The power supply switching circuit 17 is constituted by a logic circuit and is adapted to logically process the output signals F1, F2 and F3 into control signals U.sub.H, U.sub.L, V.sub.H, V.sub.L, W.sub.H and W.sub.L which are in turn are supplied to the bases of the driving transistors 10 to 15 to cause the latter to perform their respective switching operations. At this time, the switching operations are carried out so that a motor driving torque is generated in one direction at all times for driving a motor.
In the prior art construction, it is necessary to use a filtering circuit having a cut-off frequency characteristic for each phase of the driving coils and, accordingly, a number of capacitors having a high capacitance is required.
Also, where the inductance of the driving coils is high, the power supply current to be passed through the coils tends to be delayed in time after the driving transistors are switched on, and permanent magnetic fields tend to be degaussed by magnetic fields generated by the driving coils. A so-called armature reaction exists. In such case, it is well known that, when the driving coils are supplied with an electric power at such timings as shown in FIG. 18, the efficiency tends to be lowered. As a countermeasure, a technique in which the signals F1, F2 and F3 are somewhat advanced in phase to operate the driving transistors so as to compensate for a delay in power supply resulting from the armature reaction is disclosed in the Japanese Laid-open Patent Publication No. 52-80145, however, component parts, such as capacitors, are further required. Also, since the power supply waveforms Uo, Vo and Wo tend to be accompanied by spike noises generated when the driving transistors are switched off, a variation in power source voltage, a variation in current attributable to a change in load, and so on, it is often difficult to obtain accurately a power supply switching signal even though the power supply waveforms Uo, Vo and Wo are inputted to the filtering circuit. As a countermeasure, a system has been suggested such as disclosed in the Japanese Patent Publication No. 59-36519.
However, the system using the filtering circuit for providing the power supply switching signal from the power supply waveform for the driving coils basically has the following problem. That is, a voltage drop resulting from both the power supplied during the supply of electric power to the driving coils and the internal impedances of the driving coils, a spike noise occurring immediately after the interruption of the power supply, and so on, tend to be superimposed on a fundamental wave (counterelectromotive force) of the power supply waveforms of the driving coils. Such a voltage drop and spike constantly vary with a variation in power source voltage and load. Accordingly, where the power supply waveforms of the driving coils are filtered to provide the power supply switching signal, an error tends to occur as a result of the above described component, such as the voltage drop, spike, etc., which is superimposed on the fundamental wave (counterelectromotive force) of the power supply waveforms while the latter constantly vary, and it is therefore difficult to accurately supply the electric power to the driving coils.
In order to eliminate the above described conventional problems, various methods have been suggested to obtain the power supply switching signal accurately, all of which are basically characterized in that adjustment is effected in the periphery of the filtering circuit for maintaining at a constant value the difference in phase between the driving coil counterelectromotive force and the power supply switching signal. Such an adjustment is extremely cumbersome. Also, other than those necessary for the filtering circuit, a number of additional capacitors are required and, therefore, when the driving circuit is fabricated into an integrated circuit, both the number of component parts to be connected and the number of connection pins tend to increase, rendering the price high. Also, a system wherein no filtering circuit is employed and, instead, the use is made of, for example, a microcomputer for digitally providing the power supply switching signal is disclosed in the Japanese Laid-open Patent Publication No. 61-293191, which is relatively expensive.
As hereinbefore discussed, since the conventional drive device for the brushless motor is constructed so that the filtering circuit is used to process the power supply waveforms of the driving coils to provide the power supply switching signal having a predetermined phase relationship to the position of the movable element, the switching signal being utilized to sequentially energize the driving coils, it is not possible to obtain an accurate power supply switching signal because of a voltage drop in the driving coils resulting from the spike noises contained in the power supply waveforms of the driving coils and the electric current supplied, a variation of the superimposed component resulting from the change of the power source voltage and the load, the armature reaction and so on. Also, an increased number of capacitors having a high capacitance is required for constructing the filtering circuit and, in particular, when the driving circuit is to be fabricated in an integrated circuit, both the number of component parts to be connected and the number of connection pins tend to increase, rendering it disadvantageous in terms of price.
In view of the foregoing, a system such as disclosed in the Japanese Patent Publication No. 61-3193 is suggested wherein the counterelectromotive force generated in the driving coils is shaped as to its waveform and use is made of a phase sync loop (PLL) circuit to generate an appropriate phase pulse to permit the driving coils to be sequentially supplied with electric power to drive the motor. In other words, there is disclosed a system wherein, according to the construction shown in FIG. 19, counterelectromotive voltages A, B and C generated by the driving coils are pulse-shaped and arithmetically processed to provide a pulse signal G which is compared in phase with a divider output I provided at an output of a voltage controlled oscillator, a thus compared output being fed back to the voltage controlled oscillator to make the signals I and G to be matched in phase with each other so that the frequency division of the signal I can result in a generation of the power supply signal for the driving coils to thereby drive the motor. The behavior of the signals at various portions of this system are shown in FIG. 20. However, in such a system, a voltage drop resulting from the electric current supplied during the supply of the electric power to the driving coils and the internal impedances of the driving coils, the spike noises generated immediately after the interruption of the power supply, and so on, are superimposed on the counterelectromotive voltage generated in the driving coils as hereinbefore described and, therefore, it is extremely difficult to obtain the pulse signal by shaping and arithmetically processing the counterelectromotive force generated in the driving coils. In fact, with the construction shown in FIG. 19, it is not easy to obtain a pulse signal G as shown in FIG. 20 and the spike noise generated immediately after the power supply through the driving coils necessarily affects the operation. Accordingly, it is impossible to accomplish the phase comparison with the divider output I and, therefore, it is impossible to render the phases of the signals I and G to be matched with each other.
As hereinbefore discussed, the conventional drive devices for the brushless motor have had various problems.